Conventionally, light-emitting devices using semiconductors have been widely used for display and illumination and as light sources.
As one of such light-emitting devices, there is a light-emitting diode wherein, by applying a forward voltage to pn-junction semiconductors, electrons are injected into an n-region and holes are injected into a p-region so that light emission is achieved by recombination of electrons and holes at a junction (active region).
The pn-junction of the light-emitting diode is formed by joining together a p-type semiconductor and an n-type semiconductor each produced by doping into a semiconductor an additive that forms p-type or n-type. For example, as such a pn-junction light-emitting diode, a light-emitting diode using a diamond single crystal as an emission active material is described in Japanese Journal of Applied Physics, part 2, vol. 40, pp. L275-L278.
However, in the pn-junction light-emitting diode, it is an unavoidable problem that while sufficient light emission cannot be obtained without doping a lot of additives into semiconductor materials, there occur deformation, defect, and the like in crystal structures of the semiconductor materials due to the doping of the additives, which become quenching centers or induce light emission of unnecessary wavelengths, thereby reducing the efficiency of contribution of the injected carriers to light emission.
On the other hand, in contrast to the foregoing pn-junction light-emitting device, there is known a field-effect transistor type light-emitting device as a light-emitting device of another type using an organic substance. A field-effect transistor type light-emitting device (hereinafter A-LEFET) using α-sexithiophen is disclosed in Science Journal, vol. 290, Nov. 3, 2000, pp. 963-965.
FIG. 3 is a diagram showing a structure of a field-effect transistor type light-emitting device using α-sexithiophen.
The foregoing A-LEFET using α-sexithiophen has a structure wherein a single crystal of α-sexithiophen (hereinafter α-6T) formed by the vacuum evaporation method is used for an emission active member 21 which is sandwiched between aluminum-deposited films as a source electrode 23 and a drain electrode 24 formed with an interval of 25 μm therebetween. Further, a gate electrode 27 in the formed of an aluminum-doped zinc oxide (ZnO:Al) film is formed on the α-6T, the source electrode, and the drain electrode via aluminum oxide (Al2O3) as a gate insulating layer 26 sandwiched therebetween, thereby forming a metal/insulator/semiconductor (hereinafter MIS) type field-effect transistor (hereinafter FET) structure.
In α-6T, an intrinsic crystal thereof simultaneously has both an electron transporting property and a hole transporting property. That is, this material exhibits a so-called ambipolar conduction property by being given an injection mechanism for electrons and holes.
By applying positive potential, as compared to the potential at the source electrode as a reference, to the drain electrode and the gate electrode of the foregoing A-LEFET, electrons and holes are injected into an electron conduction level and a hole conduction level of α-6T from the source electrode and the drain electrode, respectively. These positive and negative charged carriers cause radiative recombination transition at the middle portion of a conducting channel to generate electroluminescence, thereby emitting light.
In the A-LEFET disclosed in the foregoing literature, inasmuch as the α-6T single crystal having both the hole transporting property and the electron transporting property is used for the emission active member, it is not necessary to dope into the emission active material the additives which are required for forming the pn-junction and injecting minority carriers into the light-emission region in case of the pn-junction type. Therefore, the foregoing problem of the reduction in light-emission property due to the additives is not raised.
(Problem to be Solved by the Invention)
However, there are the following problems even in the foregoing A-LEFET.
Although the single-crystal organic substance like α-6T is used for the emission active member, the mobility of electrons and the mobility of holes in the organic substance are generally small so that it is difficult to obtain a sufficient light-emission intensity. A typical numerical value of the mobility at room temperature is 10−3 to 10−4 cm2/(V·s), and about 100 cm2/(V·s) at maximum.
In addition, it has been difficult, in the first place, to form a faultless single crystal in the organic substance, and therefore, difficult to perform efficient light emission. Further, in general, light-emitting devices are unavoidably subjected to heat generation, and there has been a problem that the A-LEFET type light-emitting device using the organic substance like α-6T is unavoidably subjected to deterioration with age in environment of use so that it is difficult to achieve a long-term reliability against deterioration.
Further, in case of the A-LEFET type light-emitting device where the emission active member is the organic substance, the spectrum of generated electroluminescence is defined, according to the light-emission mechanism, only by an energy difference between recombination levels of the emission active member into which charges can be injected from the source electrode and the drain electrode. Therefore, when α-6T is used, the emission wavelength is limited to a photon energy region of 1.6 eV or more to 2.2 eV or less (corresponding to wavelength of about 564 to 775 nm). Accordingly, in the A-LEFET type light-emitting device where the emission active member is the organic substance, the emission wavelength is determined by a substance selected for the emission active member. However, when the organic crystalline substance is used for the emission active member, it is difficult to select a substance having ambipolarity necessary for obtaining a desired wavelength, for example, light of a shorter wavelength region, and therefore, there has been a problem of poor flexibility in emission wavelength.
In view of the foregoing problems, it is an object of the present invention to provide a light-emitting device that can obtain a long-term reliability and broaden a selectivity of emission wavelength.